The present invention relates to photothermographic materials exhibiting superior storage stability and in particular to black-and-white photothermographic materials exhibiting superior storage stability of silver images.
In the field of medical treatment and graphic arts, there have been problems in working property with respect to effluents produced from wet-processing of image forming materials, and recently, reduction of the processing effluent is strongly demanded in terms of environment protection and space saving.
Accordingly, there are needed techniques regarding photothermographic materials for photographic use and which are capable of forming black images exhibiting high sharpness, enabling efficient exposure by means of a laser imager or laser image setter. As such a technique is known a photothermographic material, which comprises a support having thereon an organic silver salt, light-sensitive silver halide grains and a reducing agent, as described in U.S. Pat. No. 3,152,904 and 3,487,075; and D. Morgan xe2x80x9cDry Silver Photographic Materialxe2x80x9d (Handbook of Imaging Materials, Marcel Dekker, Inc., page 48, 1991). No processing solution is used in this photothermographic material (hereinafter, also referred to as a photothermographic material), enabling a simple system friendly to the environment and operators.
Since this thermally developable photothermographic material contains an organic silver salt, light-sensitive silver halide grains and a reducing agent, there are problems such that the photothermographic material not only tends to cause fogging before or during thermal development but also easily produces fog or photolytic silver (or print-out silver). Specifically, this photothermographic material, after exposure, is subjected only to thermal development at a temperature of 80 to 250xc2x0 C., without being further subjected to fixing so that there were such problems that silver images causes discoloring upon exposure to light or heat during storage under the concurrent presence of the silver halide, organic silver salt and reducing agent which remained in unexposed areas. It is contemplated that the presence of the reducing agent in the photothermographic material easily results in formation of fog upon reaction with the organic silver salt and that when exposed to light having different wavelengths from light employed in image recording after processing, the reducing agent functions as a hole-trap in addition to its inherent function to reduce silver ions, leading to enhanced print-out from the silver halide or organic silver salt. In addition to the foregoing causes, it is contemplated that fog specs causing fogging are formed in the course of manufacturing the photothermographic material.
A technique for solving these problems is disclosed in JP-A 6-208192 and 8-267934 (hereinafter, the term, JP-A means an unexamined and published Japanese Patent Application) and references cited therein. Although these disclosed techniques were effective to some extent, they were not sufficiently at levels required by the market.
In view of the foregoing, the present invention was made and it is therefore an object of the present invention to provide a photothermographic material exhibiting little fogging even after storage for a long period of time and superior silver image stability after thermal processing.
The above object of the invention can be accomplished by the following constitution:
1. A method for preparing a photothermographic material comprising an organic silver salt, wherein the method comprises a step of treating the organic silver salt under a gas atmosphere containing an inert gas having a volume fraction of not less than 85% or under a gas atmosphere containing oxygen gas having a volume fraction of not more than 15%;
2. The method described in 1, comprising the steps of:
(a) preparing an organic silver salt,
(b) preparing an emulsion containing the organic silver salt and a silver halide,
(c) drying the emulsion,
(d) coating the emulsion, and
(e) drying the coated emulsion,
xe2x80x83and wherein at least one of the steps (a) through (e) is conducted under the gas atmosphere containing an inert gas having a volume fraction of not less than 85% or under a gas atmosphere containing oxygen gas having a volume fraction of not more than 15%;
3. The method described in 2, wherein step (c) is conducted under the gas atmosphere containing an inert gas having a volume fraction of not less than 85% or under a gas atmosphere containing oxygen gas having a volume fraction of not more than 15%;
4. The method described in 1, wherein the inert gas is at least one selected from the group consisting of nitrogen, helium and argon;
5. The method described in 3, wherein in step (c), the emulsion is dried at a temperature of 35 to 80xc2x0 C.;
6. The method described in 1, wherein the photothermographic material further comprises a light sensitive silver halide, a reducing agent and a binder;
7. The method described in 6, wherein the photothermographic material further comprises a cross-linking agent;
8. The method described in 7, wherein the photothermographic material further comprises a compound capable of generating a labile species other than a halogen atom upon exposure to ultraviolet ray or visible light to deactivate the reducing agent;
9. The method described in 8, wherein the labile species other than a halogen atom is a free radical comprised of plural atoms;
10. The method described in 7, wherein the cross-linking agent is selected from the group consisting of an expoxy compound, acid anhydride, an isocyanate compound, and an isothiocyanate compound;
11. A method of preparing a package containing a photothermographic material comprising an organic silver salt, wherein the method comprises a step of treating the organic silver salt under a gas atmosphere containing an inert gas having a volume fraction of not less than 85% or under a gas atmosphere containing oxygen gas having a volume fraction of not more than 15%;
12. The method described in 11, comprising the steps of:
(a) preparing an organic silver salt,
(b) preparing an emulsion containing the organic silver salt and a silver halide,
(c) drying the emulsion,
(d) coating the emulsion,
(e) drying the coated emulsion to prepare a photothermographic material, and
(f) packaging the photothermographic material to prepare a package containing the photothermographic material and wherein at least one of the steps (a) through (f) is conducted under the gas atmosphere containing an inert gas having a volume fraction of not less than 85% or under a gas atmosphere containing oxygen gas having a volume fraction of not more than 15%;
13. The method described in 12, wherein step (f) is conducted under the gas atmosphere containing an inert gas having a volume fraction of not less than 85% or under a gas atmosphere containing oxygen gas having a volume fraction of not more than 15%;
14. The method described in 11, wherein the inert gas is at least one selected from the group consisting of nitrogen, helium and argon;
15. The method described in 11, wherein the photothermographic material further comprises a light sensitive silver halide, a reducing agent and a binder;
16. A package containing a photothermographic material, wherein the package is filled with a gas containing an inert gas having a volume fraction of not less than 85% or with a gas containing oxygen gas having a volume fraction of not more than 15%;
17. The package described in 16, wherein the inert gas is at least one selected from the group consisting of nitrogen, helium, and argon;
18. The package of claim 16, wherein the package further contains a deoxidant;
19. The package described in 18, wherein the deoxidant is at least one selected from the group consisting of ferrous salts, iron powder, sulfites, hydrogen sulfites, dithionites, hydro uinone, catechol, resorcinol, pyrogallol, gallic acid, Rongalit, ascorbic acid, ascorbates, isoascorbic acid, isoascorbates, sorbose, glycose, lignin, dibutylhydroxytoluene and butylhydroxyanisole.